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The reaction force F( ξ) is the negative gradient of the potential energy of a chemical process along the intrinsic reaction coordinate ξ. We extend the rigorous concept of F( ξ) to the 'activation strain model' of Bickelhaupt et al., to formulate the 'strain' force F( ξ) that retards a reaction and the 'interaction' force F( ξ) that drives it. These are investigated for a group of Diels-Alder cycloadditions. The results fully support the interpretation of the minimum of F( ξ) as defining the beginning of the transition from deformed reactants to eventual products. [ABSTRACT FROM AUTHOR]

A synchronous, concerted chemical process is rigorously divided by the reaction force F( R), the negative gradient of V( R), into 'reactant' and 'product' regions which are dominated by structural changes and an intervening 'transition' region which is electronically intensive. The reaction force constant κ( R), the second derivative of V( R), is negative throughout the transition region, not just at the nominal transition state, at which κ( R) has a minimum. This is consistent with experimental evidence that there is a transition region, not simply a specific point. We show graphically that significant nonsynchronicity in the process is associated with the development of a maximum of κ( R) in the transition region, which increases as the process becomes more nonsynchronous. (We speculate that for a nonconcerted process this maximum is actually positive.) Thus, κ( R) can serve as an indicator of the level of nonsynchronicity. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]